Support for Enum type in MySQL adapter. Requires patch to mainline JDB.

(See pull request: https://github.com/1602/jugglingdb/pull/296.)

After this there are several ways to use Enum with the adapter as shown in datatypes.test.js.
After feedback changed Enum implementation to be a function.
The main side effect is that the `Enum` type can't be registered.
Use `EnumFactory()` to build an `Enum`.
Merge pull request #39 from dgsan/master

Support for Enum type in MySQL adapter. Requires patch to mainline JDB.